Add missing license headers (and support easy means to add headers in the future)

This configures Mycila's license-maven-plugin and uses it to add missing license headers:

http://code.mycila.com/license-maven-plugin/
http://code.mycila.com/license-maven-plugin/reports/2.3/plugin-info.html

* License header template stored in header.txt
* Only applied to Java files, with munged/lib/test/example files ignored
* Skips any existing (javadoc-style) header files

Use 'mvn license:format -N' at top of project to add missing headers